Hi,

I'm a latecomer to the party and didn't realize there had already been healthy debate, but here's what I came up with.

Looking through the previous submissions above, it's the same concept as Brendan's, but extended to support the full functionality of the original post.

For the original example (working on an unoptimized temp table) the time taken seems to be approx 0.25 secs/1000 records, so with 10000 records I'm getting something like 3 seconds run time. With the code below there seems to be no increase in time taken at all (around 0.25 secs for anything from 0 to 10000 records), but this is probably because for small datasets the main delay below is not due to the work itself, but rather the creation of the temp table.

I'd be interested to see if anyone can make something faster! (ignoring the fact that I could probably have made it a table variable...)

Thanks,

Tao

CREATE TABLE #StartsAndEnds (EventID Int, EventType NVarChar(20), IdentityTracker INT IDENTITY (1,1))
INSERT INTO #StartsAndEnds (EventID, EventType)
SELECT SeqNumber, Type
FROM (
 SELECT Seq1.SeqNumber, 'Start' AS Type
 FROM #SequenceTable Seq1
 LEFT JOIN #SequenceTable Seq2 ON Seq2.SeqNumber = Seq1.SeqNumber + 1
 WHERE Seq2.SeqNumber Is Null
 
 UNION ALL
 
 SELECT Seq1.SeqNumber, 'End' AS Type
 FROM #SequenceTable Seq1
 LEFT JOIN #SequenceTable Seq2 ON Seq2.SeqNumber = Seq1.SeqNumber - 1
 WHERE Seq2.SeqNumber Is Null
) AS PingPong
ORDER BY SeqNumber

SELECT IsNull(FL1.EventID, 0) AS StartGap,
 FL2.EventID As EndGap,
 IsNull(FL1.EventID, 0) + 1 AS FirstAvailable,
 FL2.EventID - 1 AS LastAvailable,
 FL2.EventID - IsNull(FL1.EventID, 0) - 1 As AvailableCount
FROM #StartsAndEnds FL2
LEFT JOIN #StartsAndEnds FL1 ON FL2.IdentityTracker = FL1.IdentityTracker + 1
WHERE FL2.IdentityTracker % 2 = 1

DROP TABLE #StartsAndEnds

Hi Tao, did you post the correct version of your code?

I copied your code above and did a find/replace on #SequenceTable and SeqNumber swapping these for my table name and sequence field respectively, then ran it.  It was certainly quick, very impressive, 16 seconds to produce 142874 identified breaks in the sequence from a table containing 3,827,625 rows. 

First few rows of output are as below

My original query over the same table takes 44 seconds and produces the following first few rows.

I've checked the results and my query is showing the correct gaps.  Your result line showing 58224 to 62107 represents my end gap result line 5 above to my start gap result line 6 above.

Love to see your revision as certainly looks like it will be very fast.

Stephen

-- Drop procedure if it exists
IF EXISTS (SELECT * FROM [sys].[objects] WHERE OBJECT_ID = OBJECT_ID(N'[dbo].[FindGap]') AND [type] IN (N'P', N'PC')) DROP PROCEDURE [dbo].[FindGap]
GO

-- Create procedure
CREATE PROCEDURE [dbo].[FindGap]
   @strFindGapTable         VARCHAR(254),
   @strFindGapField         VARCHAR(254),
   @binFindGapLower         BIGINT = 1,
   @intFindGapMode          INT = 0   
AS
-- +---------------------------------------------------------------------------+
-- |                                                                           |
-- | Type    : SQL Procedure                                                   |
-- | Name    : FindGap                                                         |
-- | Version : 1.1                                                             |
-- | Creator : Martin Grape, ActionBase, +46 73 391 88 89                      |
-- | Info    : Returns first available numer in a field of sequence or         |
-- |         : displays a list of available gaps and number of gaps.           |
-- | Input   : @strFindGapTable -> Name of table to run on                     |
-- |         : @strFindGapField -> Name of field                               |
-- |         : @strFindGapLower -> Lower value to find gap from                |
-- |         : @strFindGapMode -> 0=All gaps, 1=Next available number          |
-- | Example : EXEC FindGap 'yourtable', 'sysid', 1, 1                         |
-- | History : 2011-07-03 Created v1.0                                         |
-- |         : 2011-07-03 Added check for all numbers in sequence v1.1         |
-- |         :                                                                 |
-- |                                                                           |
-- +---------------------------------------------------------------------------+

-- Declare variables
DECLARE @strFindGapSQL    AS NVARCHAR(2000);
DECLARE @strFindGapPar    AS NVARCHAR(2000);
DECLARE @binFindGapMin    AS BIGINT;
DECLARE @binFindGapMax    AS BIGINT;
DECLARE @binFindGapCount  AS BIGINT;
DECLARE @binFindGapFirst  AS BIGINT;

-- Make SQL to find min and max values
SET @strFindGapSQL = N'SELECT @binMinVal = MIN(' + @strFindGapField + '), @binMaxVal = MAX(' + @strFindGapField + '), @binCount = COUNT(*) FROM ' + @strFindGapTable;
SET @strFindGapPar = N'@binMinVal BIGINT OUTPUT, @binMaxVal BIGINT OUTPUT, @binCount BIGINT OUTPUT';

-- Get values into variables
EXEC SP_EXECUTESQL  @strFindGapSQL, @strFindGapPar, @binFindGapMin OUTPUT, @binFindGapMax OUTPUT, @binFindGapCount OUTPUT;

-- Check mode
IF @intFindGapMode = 1
   -- Next available sequence number
   BEGIN
      -- Check if min is bigger than supplied lower, then take that value
      IF @binFindGapMin > @binFindGapLower 
         BEGIN   
            RETURN @binFindGapLower;
         END
      -- Check if max is smaller than supplied lower, then take that value
      ELSE IF @binFindGapMax < @binFindGapLower 
         BEGIN
            RETURN @binFindGapLower;
         END
      -- Check if max is the same as supplied lower, then return + 1
      ELSE IF @binFindGapMax = @binFindGapLower 
         BEGIN
            RETURN @binFindGapLower + 1;
         END
      -- Check if no existing gaps, return higest + 1
      ELSE IF @binFindGapCount = @binFindGapMax - @binFindGapMin + 1
         BEGIN
            RETURN @binFindGapMax + 1
         END
      -- Get first available number   
      ELSE      
         BEGIN      
            -- Create SQL to get next value 
            SET @strFindGapSQL = N'SELECT TOP 1 @binMinVal = LastSysId + 1 FROM (SELECT (SELECT TOP 1 ' + @strFindGapField + ' FROM ' + @strFindGapTable + ' WHERE ' + @strFindGapField + ' < a.' + @strFindGapField + ' ORDER BY ' + @strFindGapField + ' DESC) AS LastSysId, a.' + @strFindGapField + ' AS NextSysId FROM ' + @strFindGapTable + ' AS a LEFT JOIN ' + @strFindGapTable + ' AS b ON a.' + @strFindGapField + ' = b.' + @strFindGapField + ' + 1 WHERE b.' + @strFindGapField + ' IS NULL) AS a WHERE LastSysId IS NOT NULL ORDER BY 1';
            SET @strFindGapPar = N'@binMinVal BIGINT OUTPUT';
            -- Get value into variable
            EXEC SP_EXECUTESQL  @strFindGapSQL, @strFindGapPar, @binFindGapFirst OUTPUT;
            -- Return value
            RETURN @binFindGapFirst;
         END   
   END 
ELSE
   -- All available sequence numbers as a list
   BEGIN
      -- Check if first value is bigger than supplied lower, then add a row
      IF @binFindGapLower < @binFindGapMin
         BEGIN
            -- Add a union from supplied lower to the first actual value
            SET @strFindGapSQL = N'SELECT ' + CAST(@binFindGapLower AS NVARCHAR) + ' AS GapFrom, ' + CAST(@binFindGapMin - 1 AS NVARCHAR) + ' AS GapTo, ' + CAST(@binFindGapMin - @binFindGapLower AS NVARCHAR) + ' AS GapSize UNION ';
         END   
      ELSE
         BEGIN
         -- Reset variable
         SET @strFindGapSQL = '';
      END                  
      -- Check if to add 
      SET @strFindGapSQL = @strFindGapSQL + 'SELECT LastSysId + 1 AS GapFrom, NextSysId - 1 AS GapTo, NextSysId - (LastSysId + 1) AS GapSize FROM (SELECT (SELECT TOP 1 ' + @strFindGapField + ' FROM ' + @strFindGapTable + ' WHERE ' + @strFindGapField + ' < a.' + @strFindGapField + ' ORDER BY ' + @strFindGapField + ' DESC) AS LastSysId, a.' + @strFindGapField + ' AS NextSysId FROM ' + @strFindGapTable + ' AS a LEFT JOIN ' + @strFindGapTable + ' AS b ON a.' + @strFindGapField + ' = b.SysId + 1 WHERE b.' + @strFindGapField + ' IS NULL) AS a WHERE LastSysId IS NOT NULL ORDER BY 1';
      -- Run the select
      EXEC (@strFindGapSQL);
      -- Return also the rowcount
      RETURN @@ROWCOUNT;
   END
-- +------------------+
-- | END OF PROCEDURE |
-- +------------------+
GO

-- +---------+
-- | Testing |
-- +---------+

-- Drop test table if it existst
IF EXISTS (SELECT * FROM [sys].[objects] WHERE OBJECT_ID = OBJECT_ID(N'[dbo].[yourtable]') AND [type] IN (N'U')) DROP TABLE [dbo].[yourtable]
GO


-- Add dummy data
SELECT 2 AS SysId INTO yourtable UNION
SELECT 6 AS SysId UNION
SELECT 8 AS SysId UNION
SELECT 9 AS SysId UNION
SELECT 11 AS SysId;


-- Example 1: Adds a row with next available number starting on 0 , displays the result table and selects next available number
DECLARE @binFindGapReturn BIGINT;
EXEC @binFindGapReturn = FindGap 'yourtable', 'sysid', 0, 1;
INSERT INTO yourtable (SysId) SELECT @binFindGapReturn
SELECT * FROM yourtable ORDER BY 1
EXEC @binFindGapReturn = FindGap 'yourtable', 'sysid', 0, 1;
SELECT @binFindGapReturn AS NextValue;
GO


-- Example 2: Returns all the gaps starting on 1 as a resultset and returns number of gaps
DECLARE @binFindGapReturn BIGINT;
EXEC @binFindGapReturn = FindGap 'yourtable', 'sysid', 1, 2;
SELECT @binFindGapReturn AS NoOfGaps;
GO

-- Example 3: Fill missing sequences to get max + 1
INSERT INTO yourtable SELECT 1
INSERT INTO yourtable SELECT 3
INSERT INTO yourtable SELECT 4
INSERT INTO yourtable SELECT 5
INSERT INTO yourtable SELECT 7
INSERT INTO yourtable SELECT 10
DECLARE @binFindGapReturn BIGINT;
EXEC @binFindGapReturn = FindGap 'yourtable', 'sysid', 0, 1;
SELECT @binFindGapReturn AS NextValue;
GO


-- Drop test table if it existst
IF EXISTS (SELECT * FROM [sys].[objects] WHERE OBJECT_ID = OBJECT_ID(N'[dbo].[yourtable]') AND [type] IN (N'U')) DROP TABLE [dbo].[yourtable]
GO